Low carbon high strength steel



lFiled oct. 29, 1958 May 29, 1962 G. A. ROBERTS ET AL 3,036,912

Low CARBON HIGH STRENGTH STEEL 2 Sheets-Sheet 1 AQ. 300 SOO 700 900 HOO |300 T MRF TEMPER'NG E ATTORNEY.

May 29, 1962' n G. A. ROBERTS ET AL 3,036,912

Low CARBON HIGH STRENGTH STEEL TEMPERING TEMPERATURE- F GEORGE A. ROBERTS JOHN C. HAMAKER JR.

IN VEN T ORS.

ATTORNEY.

United States Patent O 3,035,912 LOW CARBON HIGH STRENGTH STEEL George A. Roberts and John C. Hamaker, Jr., Latrobe, Pa., assignors to Vanadium Alloys Steel Company, Latrobe, Pa.

Filed Oct. 29, 1953, Ser. No. 770,376 7 Claims. (Cl. 75-126) This invention relates to a new and improved alloy steel of the same general type as that disclosed in our copending application, Serial No. 679,231, filed August 20, 1957, now abandoned. The aforesaid application discloses an alloy composition having yultra high strength properties which render it particularly suitable for use in the manufacture of missile and aircraft components and other applications requiring high strength at room as well as elevated temperatures. This alloy composition has been commercialized by Vanadium-Alloys Steel Company, the assignee of the instant application, under the trade name VascoJet 1000, which name will be hereinafter employed in referring to such composition.

While the carbon content range in our co-pending application has proved to be very satisfactory, and about the optimum for lforgings and bar stock for manufacture into airframe parts, landing gear and other high strength aircraft uses, ditliculties have been encountered with this material in sheet form due to a sensitivity to low ductility under multi-axial hydraulic stress and to welding problems. The low alloy constructional steel AISI 4130, which is most widely used in welded pressure vessels for missiles, suiers from other disadvantages. This material is a common aircraft steel and has a nominal analysis of .30% carbon, .50% manganese, .28% silicon, .95% chromium and .20% molybdenum. It is normally hardened by oil quenching from 1550 F. and is only capable of achieving about 210,000 p.s.i. tensile strength and 190,000 p.s.i. yield strength (at .2% offset) in the asquenched condition. Upon tempering, which is necessary for the development of maximum toughness and ductility, the strength properties are lowered regardless of the temperature range. This is due to the fact that a low alloy steel of this type has no secondary hardening characteristics. The typical mechanical properties of AISI 4130 oil quenched from 1550 F. are `as follows:

Furthermore, this material has very limited hardenability and must be oil quenched, whichV presents serious problems of distortion and in transferring largeobjects from the Ifurnace to the quenching bath.

We have discovered that these difficulties may be overcome by lowering the carbon rangel in Vascolet 1000 to below .25% while simultaneously maintaining the vanadium content above .35%. While the lower carbon content lowers the higher strength levels obtainable with Vascol et 1000, the improvements in ductility and weldability thus obtained olset these lower strength levels and the high vanadium `content induces strong grain refining tendencies which permit higher hardening temperatures while retaining a ne grained, tough structure. With these higher hardening temperatures, the very stable vanadium carbides are dissolved in the matrix and then precipitate on tempering at 950-1'150 F. to produce a very pro- 3,036,912 Patented May Z9, 1962 nounced secondary hardening tendency, as will be explained more fully below. The secondary hardening tendency due to alloy carbide precipitation is indicative of excellent heat resistance and hot hardness and results in a composition which after tempering may possess a yield strength in excess of 200,000 p.s.i. at .2% offset. In our new composition we have a steel capable of air hardening through very thick sections (upto about 20 cube) Without apreciable distortion. Since it is usually tempered above 950 F. 'hardening stresses are removed with ease. Our vanadium content of above .35% is very important in connection with thi-s aspect of the invention, as is the low carbon range. It is the combination of these factors in our present composition which we deem new and useful, to wit, carbon below .25% and vanadium above .35%. We have further discovered that tungsten up to 2% may, in some instances, be advantageously brought into our composition.

Critical data appearing in the specication is graphically illustrated in FIGURES 1 through 6 wherein;

FIGURE 1 is a graph showing the as-quenched hardness Iand fracture grain size of applicants steel for various hardening temperatures;

FIGURE 2 is a graph showing the hardness of a steel specimen triple tempered at l000 F. after air cooling from various hardening temperatures;

FIGURE 3 is a graph showing the hardness of a steel specimen triple tempered `at 1000 F. after oil quenching from various hardening temperatures;

FIGURE 4 is a graph -showing the hardness of a steel specimen triple tempered at various temperatures after air cooling from a hardening temperature of 1800 F.;

FIGURE 5 is a graph showing the hardness of a steel specimen triple tempered at various temperatures after air cooling from a 'hardening temperature of 1850 F.; and

FIGURE 6 is a graph showing some of the physical and engineering properties of a steel specimen triple tempered at various temperatures after air cooling from a hardening temperature of 1800 F.

`One example of our improved composition has the following chemical analysis:

with the remainder substantially all iron.

A test material of this composition was rolled starting with a 11/2 square billet. The average annealed hardness was:

179 Brinell 89 Rockwell B AUSTENITIZA'IION The specimen size used was 1/2 cubes. heat in salt at 1250 F. for l5 minutes, the samples were transferred to the austenitizing furnace. A gas-fired, semi-mufiie preheat furnace was used for 1700, 1800 and 1850" F. A gas-lired semi-muflle high heat furnace was used for 1900, 2000 and 2100 F. All specimens were austenitized for 30 minutes and oil quenched or air cooled. 0.025" of stock was removed from the surface for as-quenched hardness readings. The specimens were then nicked and fractured and the grain size determined by comparison to Shepard grain size standards. The asquenched hardness and fracture grain size are tabulated After a prebelow in Table I and are graphically illustrated in FIG- URE l.

Table I l Tempered structures (1000 or 1l00 F. triple tempered) of air cooled specimens austenitized at i809 or [Specimens air cooled or oll quenched after 30 minutos at 'the indicated austenitizing temperature and triple tempered (2+2-l-2 hours) at 1000 F. Rockwell C hardness averaged from 5 readings] Air Cooled Oil Quenelied Austenitizing Fracture Temperature, Grain F As First Second Third As First Second Third Size Quenched Temper Temper Temper- Quenelied Tcniper Teinper Tamper 32. 6 34. 4 34. 3 37. 0 3G. 2 38. 2 37. 7 37. 6 0. 9 40. 2 43. 4 42. 9 43. 3 45.0 47. 1 45. 9 45. 8 6.0 47. 7 49. 8 47. 7 5l. l 4S. 3 49. 5 48. 5 48. 9 6. 25 48. 0 50. 8 50. 9 51. 1 47. 8 5l. 0 48. S 49. 2 5. 0 47.9 50. 9 49. 2 50.4 47. 7 52. 1 50. 3 50. 8 4. 75 47. 7 50. 3 51. 2 51. 3 47. 50. 8 50. 1 50. 4 4. 5 46. l 50. G 46. 9 50. 8 45. 9 50. 1 49. 0 49. l 4. 5

TEMPERING l850 F. displayed light etching effects typical of low As a preliminary check on the eiect of tempering, the as-quenched specimens were given a triple temper (2-1-2-1-2 hours) at 1000 F. Hardness tests were taken, with 0.010" surface removal after each two hour temper. These are recorded in Table I above, and are graphically presented in FIGURES 2 and 3.

Austenitizing temperatures of 1800 and l850 F. were selected for a complete tempering study. The procedure for austenitizing was identical to that described above. All specimens were air cooled from the two selected austenitizing temperatures.

The specimens were triple tempered (2+2-l-2 hours) at 300, 500, 800, 900, 950, 1000, 1050, 1100, 1200 and 1300 F. Rockwell C hardness readings after each two hour temper are recorded below in Table II and are graphically represented in FIGURES 4 and 5. The pronounced secondary hardness peak at 950-1050 F. is probably related to the highly alloyed carbides produced by the high alloy/carbon ratio.

Table Il [Specimens air cooled after 30 minutes at 1800 or 1850 F. and triple tempered (2+2+2 hours) at the indicated teni;

peratures. Rockwell C" results are averaged from o readings] Austenitized at 1800 Austenitized at 18.50 F., As-Quenched F., As-Quenehed Tempering Hardness 47.5 Hardness 47.4 Temperature First Second Third First Second Third Temper Temper Temper Tamper Tamper Temper 47. 1 47. 5 47. 6 47. 8 47. 6 47. 5 45. 2 47. 0 4G. 8 47.5 47.8 46. 4 48. l 47. 9 47.3 47. 8 47. 5 47. 9 48. 2 48. 1 48. 5 49. 1 49. 2 49. 9 49. 7 50.0 50. 2 49. 9 51. 1 51. 8 49. 9 49. 4 49. 3 50. 1 49. 2 48. 9 48. 6 48. 3 47. 4 48. 8 49. 0 48. 3 46. 2 44. 5 42. 8 47. 5 45. l 43. 3

MICROSTRUCTURE A series of microstructures of specimens air cooled from progressively higher austenitizing temperatures displayed a low degree of solution at 1600 and 1700 F. resulting in considerable excess carbides. At 1800 F. and above, the carbides were nearly completely dissolved and the structure consisted of low carbon transformation products with some excess carbides.

In general, the structures resulting from oil quenching were virtually indistinguishable from the air cooled ones.

carbon material.

SUMMARY From the hardening and tempering study made on our composition, air cooled or oil quenched, the following observations can be made:

(l) Ari as-quenched hardness of 47-48 Rockwell C is obtained by oil quenching or air cooling from the austenitizing range of 1800-2000 F. Triple tempering at 1000 F. increases this hardness to 49-51 Rockwell C over the entire hardening range.

(2) The low carbon content results in a moderately coarse fracture grain size of 61A at 1800 F., but higher austenitizing temperatures produce only gradual coarsening to 41/2 at 2000 F.

`(3) The high vanadium content permits higher hardening temperatures under which the stable vanadium carbides are dissolved in the matrix and precipitate on tempering at 95 0-1050 F. to produce pronounced secondary hardening.

(4) The pronounced secondary hardening which occurs at 950-1050 F. produces 3 to 4 Rockwell C hardness increase over the ats-quenched condition. Good hardness persists to 1050-1100 F., above which accelerated softening to very low hardness levels is encountered.

(5) The combination of moderately high tempered hardness and low carbon content renders this steel attrae tive for welded sheet pressure vessels at the 220,000 to 260,000 p.s.i. tensile strength level.

Another investigation was conducted to determine the eiect of tempering temperature on the tensile strength, yield strength, hardness and ductility of hot rolled 0.125" sheet of our improved composition.

The test specimens were machined from hot rolled and annealed sheet 91/2 wide by 0.125 thick, having the following chemical analysis:

Percent Carbon .22 Silicon .95 Manganese .30 Sulphur .014 Phosphorous .017 Chromium 5.05 Vanadium .47 Tungsten 1.31 Molybdenum 1.32

The specimens were machined to ASTM Specification E8-54T to represent both longitudinal and transverse orientation with regard to the direction of rolling. The specimens were heat treated in molten salt as follows:

(1) Preheated at 1125 F. for 15 minutes.

(2) Austenitized at 1800 F. for 30 minutes and air cooled.

(3) Triple tempered (2|2|-2 hours) at 950, 1000, 1050, 1100 and 1150" F.

After tempering, approximately 0.017 was ground from both sides to remove scale, decarburizaton, and other surface irregularities. Ten hardness readings were taken on the grip portion of the specimens. The gp Portions were then softened to facilitate gripping in the tensile machine by 'heating in a -Lepel high frequency induction coil. Care was taken not to affect thermally the reduced section or fillets.

TEST RESULTS The average of triplicate tests, as well as the ranges of hardness, tensile and yield strengths, percent elongation in 1 and 2 gage lengths, and reduction of area are tabulated in Table III below for both specimen orientations at all tempering temperatures investigated. IFIGURE 6 graphically presents the average data plotted as a function of tempering temperature.

Table III [Standard ASTM specimens 0.090" thick tested in triplicate. Austenitize tempered in salt for 2 hours each time] (4) An optimum combination of longitudinal properties, resulting fromrtempering at 1000 F., s:

Hardness, Rockwell C 49.0 Tensile strength p.s.i 248,000 Yield strength (0.2% olfset) p.s.i .225,000 Reduction of area percent 39.2 Elongation in 2 do 6.8 Elongation in 1" do 12.3

d minutes in salt at 1800 F. and air cooled. Triple 0.2% Offset Yield Tensile Strength Red. of Area, Rock- Strength, 1000 1000 p.s.i. Elongation in 2" Elongation in 1" percent Tempering Sp. Well p.s.l. Y Temperature, Or.l Hard- F. ness "C Aver- Range Aver- Range Aver- Range Aver- Range Aver- Range age age age age age 950 {L 51. 0 224 224-225 261 260-261 7. 7 7. 5-7. 8 12. 5 11. 5-13. 5 34. 6 33. 5-36. 5 T 51.0 224 220-226 261 260-262 6. 6 6. 5-6. 8 11.3 11. 0-11. 5 24. 0 22. 6-26. 2 1 000 {L 49. 0 225 218-231 248 247-249 6. 8 6.5-7.0 12.3 12. 0-12. 5 39.2 37. 6-41. 2 T 49. 3 221 217-224 248 246-249 5. 9 5. 8-6. 0 10. 3 10. 0-10. 5 27. U 23. 9-30. 4 1 050 {L 47. 3 207 205-211 230 228-233 7. 1 6. 5-7. 3 l2. 5 12. 5 35. 4 34. 6-36. 2 T 47. 3 207 20G-209 228 227-229 6. 7 6. 5-7. 0 1l. 2 10. 5-11. 5 28. 9 24. 3-31. 7 1 100 {L 41. 5 172 171-174 191 190-193 8.6 8. 0-9. 3 13.8 13. 5-14. 0 41. 0 40. 0-42. 0 T 41. 3 174 172-175 189 186-192 8. 3 8.0-8.5 13.3 13. 0-13. 5 32. 4 27. 4-36. 2 1 150 {Ir 32. 9 126 125-129 152 151-153 9. 4 8.7-9.8 15.0 15. 0 38. 3 37.1-40.1 T 32.6 131 13G-131 154 153-155 9. 6 8. 8-10. 0 15.3 15 0-16. 0 36. 9 35. 0-38. 4

1 Specimen Orientation. lf-Longitndnal.

T-Transverse.

The curves of hardness and tensile strength decrease linearly through the 1050 F. temper, beyond which the values decrease at an accelerated rate. The yield strength closely parallels the tensile strength, being 17 to 25 k.s.i. lower, except at the 950 F. temper, where the dilerence is 37 k.s.i Longitudinal and transverse values are virtually identical and are represented by single curves.

The percent elongation in either gage length shows a moderate increase as the tempering temperature is increased. A slight minimum occurs with the 1000 F. temper. The elongation of the longitudinal specimens is equal to or slightly superior to the transverse yspecimens at all tempering temperatures investigated.

SUMMARY From the tensile tests on 0.125l sheet of our composition, austenitized at 1800 F., lair cooled and tempered at 950 to 1150" F., the following conclusions can be drawn:

1) Tensile strengths of 150,000 to 260,000 p.s.i are attained by selection of the tempering temperature. The yield strength (0.2% offset) closely parallels the tensile ultimate, averaging 22,000 p.s.i lower, except at the 950 F. temper, Where the differential is 37,000 p.s.i

(2) Ductility (based on percent reduction of area) is up to 25% better in the range of 250,000 psi. ultimate, than the higher carbon, higher maximum strength sheet materials previously investigated.

(3) Percent elongation in the longitudinal direction is moderately superior to elongation in the transverse direction, While reduction of area is considerably superior in the longitudinal direction.

into sheet, for missile applications, having the following chemical analysis:

This material may also be used in tool and die Work as Well as for ultra high strength structural parts at room and elevated temperatures.

From. FIGURES 4 and 5' of the drawings it will be observed that the composition has a Very pronounced secondary hardening peak, which surpasses the asquenched hardness to a greater extent even than Vasco- Jet 1000. This hardness is retained to l050-1 100 F. before rapid overtemperng occurs. The sheet tensile properties indicate elongation values about equivalent to Vasco] et 1000 for a given tensile and yield strength level, but somewhat superior reduction of area which may be significant in multi-axial stress applications. The maximum strength obtainable is limited to about 255,000 p.s.i., due to the lower carbon content. However, the reduction in carbon to about one half the percentage disclosed in our co-pending application shows greatly improved resistance to micro-cracking and weld hardening that tend to appear when the higher carbon ultra high strength materials are not given sufficient care in thermal stress 7 relief and softening treatments following the welding operation.

On the basis of the foregoing our improved composition lies within the following range:

Carbon .12 to less than .25%. Silicon .80 to 1.20%. Manganese .20 to .40%.

Chromium 4.75 to 5 .25%. Molybdenum 1.20 to 1.40%. Vanadium greater than .35 to .60%.

and the remainder mainly iron, except for the permissible addition of tungsten up to 2%.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent of the United States is:

1. An alloy steel composition of the character described consisting essentially of from 0.80 to 1.20% silicon, from 0.20 to 0.40% manganese, from 4.75 to 5.25% chromium, from 1.20 to 1.40% molybdenum, from 0.12 to less than 0.25% carbon, and greater than 0.35 to 0.60% vanadium, with the remainder substantially all iron, said alloy being characterized in that on tempering at 950-115`0 F., a pronounced secondary hardening will be produced to increase the heat resistance and the hot hardness of said composition.

2. An alloy steel composition of the character described consisting essentially of from 0.80v to 1.20% silicon, from 0.20 to 0.40% manganese, from 4.75 to 5.25% chromium, from 1.20 to 1.40% molybdenum, from 0.12 to less than 0.25% carbon, from greater than 0.35 to 0.60% vanadium, and tungsten up to 2.00%, with the remainder substantially all iron, said alloy being characterized in that on tempering at 9501150 F., a pronounced secondary hardening will be produced to increase the heat resistance and the hot hardness of said composition.

3. An alloy steel composition of the character described consisting essentially of from 0.12 to less than 0.25% carbon, from 0.80 to 1.20% silicon, from .20 to .40% manganese, from 4.75 to 5.25% chromium, from 1.20

Q La

to 1.40% molybdenum, and greater than 0.35 to 0.60% vanadium, with the remainder substantially all iron.

4. The alloy according to claim 3 further defined in that said composition is heat treated at a hardening temperature of from 1600 to 2100 F., cooled and thereafter tempered at a temperature from 300 to 1300 F.

5. An alloy steel composition of the character described consisting essentially of from 0.12 to less than 0.25% carbon, from 0.80 to 1.20% silicon, from 0.20 to 0.40% manganese, from 4.75 to 5.25 chromium, from 1.20 to 1.40% molybdenum, greater than 0.35 to 0.60% vanadium, and tungsten up to 2.00%, with the remainder substantially all iron.

6. An alloy steel composition of the character described consisting essentially of from 0.19 to.0.22% carbon, from 0.82 to 0.95% silicon, from 0.27 to 0.31% manganese, from 5.05 to 5.17% chromium, from 0.47 to 0.52% vanadium, and from 1.21 to 1.38% molybdenum with the remainder substantially all iron.

7. An alloy steel composition of the character described consisting essentially of from 0.19 to 0.22% carbon, from 0.82 to 0.95% silicon, from 0.27 to 0.31% manganese, from 5.05 to 5.17% chromium, from 0.47 to 0.52% vanadium, from 1.21 to 1.38% molybdenum and from 1.28 to 1.31% tungsten with the remainder substantially all 1ron.

References Cited in the tile of this patent UNITED STATES PATENTS 1,496,979 Corning June 10, 1924 2,837,421 Kron June 3, 1958 FOREIGN PATENTS 338,556 Italy Mar. 31, 1936 OTHER REFERENCES Hildorf et al.: Transactions of ASM, vol. 27, 1939, pp. 1090-1114, published by the American Society for Metals, Cleveland, Ohio. 

2. AN ALLOY STEEL COMPOSITION OF THE CHARACTER DESCRIBED CONSISTING ESSENTIALLY OF FROM 0.80 TO 1.20% SILICON, FROM 0.20 TO 0.40% MANGANESE, FROM 4.75 TO 5.25% CHROMIUM, FROM 1.20 TO 1.40% MOLYBDENUM, FROM 0.12 TO LESS THAN 0.25% CARBON, FROM GREATER THAN 0.35 TO 0.60% VANADIUM, AND TUNGSTEN UP TO 2.00%, WITH THE REMAINDER SUBSTANTIALLY ALL IRON, SAID ALLOY BEING CHARACTERIZED IN THAT 